1) Field of the Invention
The present invention relates to a reactor system and to a process for the catalytic polymerization of olefins, and to the use of such reactor system for catalytic polymerization of olefins.
2) Description of Related Art
The catalytic polymerization of olefins uses among others a catalyst of the Ziegler-Natta type. New generations of catalyst for olefin polymerization including single site catalysts have been developed in view of a more regular polymer structure. During the catalytic polymerization the olefin composition is substantially constant around the particle. Reactor gasses exiting the reactor are compressed and recycled. Make-up monomers and optionally hydrogen are added as needed. Entrained particles can be separated by an interposed cyclone and recycled to the polymerization reactor.
Polyolefins produced using a recent generation of Ziegler-Natta catalysts have a relative narrow molecular weight distribution. The breadth of the molecular weight distribution influences the rheology behaviour and the final mechanical properties of the produced polyolefins.
In order to obtain a broader multimodal molecular weight distribution, use is made of different reactor conditions, such as different concentrations for monomer, comonomer and/or hydrogen. Another option is the use of specific and/or combined catalysts.
Traditionally, cascaded reactors are used for applying different reaction conditions during the catalytic polymerization in order to obtain a broad or multimodal molecular weight distribution.
One such type of reactor is a fluidized bed gas phase reactor. In a fluidized bed gas phase reactor olefins are polymerized in the presence of a polymerization catalyst in an upwards moving gas stream. The reactor is typically a vertical cylindrical vessel containing the fluidized bed. The bed comprises growing polymer particles containing active catalyst dispersed therein. The polymer bed is fluidized with the help of the fluidization gas comprising the olefin monomer, eventual comonomer(s), eventual chain growth controllers or chain transfer agents, such as hydrogen, and eventual inert gas. The fluidization gas is introduced into an inlet chamber at the bottom of the reactor. To make sure that the gas flow is uniformly distributed over the cross-sectional surface area of the inlet chamber the inlet pipe may be equipped with a flow dividing element as known in the art, e.g. U.S. Pat. No. 4,933,149 and EP 684871.
Traditionally from the inlet chamber the gas flow is passed upwards through a fluidization grid into the fluidized bed. The purpose of the fluidization grid is to divide the gas flow evenly through the cross-sectional area of the bed. Sometimes the fluidization grid may be arranged to establish a gas stream to sweep along the reactor walls, as disclosed in WO 2005/087361. Other types of fluidization grids are disclosed, among others, in U.S. Pat. No. 4,578,879, EP 600414 and EP 721798. An overview is given in Geldart and Bayens: The Design of Distributors for Gas-fluidized Beds, Powder Technology, Vol. 42, 1985.
The fluidization gas passes through the fluidized bed. The superficial velocity of the fluidization gas must be higher than minimum fluidization velocity of the particles contained in the fluidized bed, as otherwise no fluidization would occur. On the other hand, the velocity of the gas should be lower than the onset velocity of pneumatic transport, as otherwise the whole bed would be entrained with the fluidization gas. The minimum fluidization velocity and the onset velocity of pneumatic transport can be calculated when the particle characteristics are known by using common engineering practise. An overview is given, among others in Geldart: Gas Fluidization Technology, J. Wiley & Sons, 1986.
When the fluidization gas is contacted with the bed containing the active catalyst the reactive components of the gas, such as monomers and chain transfer agents, react in the presence of the catalyst to produce the polymer product. At the same time the gas is heated by the reaction heat.
The unreacted fluidization gas is removed from the top of the reactor, compressed and cooled in a heat exchanger to remove the heat of reaction. The gas is cooled to a temperature which is lower than that of the bed to prevent the bed from overheating because of the reaction. It is possible to cool the gas to a temperature where a part of it condenses. When the liquid droplets enter the reaction zone they are vaporised. The vaporisation heat then contributes to the removal of the reaction heat. This kind of operation is called condensed mode and variations of it are disclosed, among others, in WO 2007/025640, U.S. Pat. No. 4,543,399, EP 699213 and WO 94/25495. It is also possible to add condensing agents into the recycle gas stream, as disclosed in EP 696293. The condensing agents are non-polymerisable components, such as propane, n-pentane, isopentane, n-butane or isobutane, which are at least partially condensed in the cooler.
Prior to the entry into the reactor fresh reactants are introduced into the fluidization gas stream to compensate for the losses caused by the reaction and product withdrawal. It is generally known to analyse the composition of the fluidization gas and introduce the gas components to keep the composition constant. The actual composition is determined by the desired properties of the product and the catalyst used in the polymerization.
The catalyst may be introduced into the reactor in various ways, either continuously or intermittently. Among others, WO01/05845 and EP 499759 disclose such methods. Where the gas phase reactor is a part of a reactor cascade the catalyst is usually dispersed within the polymer particles from the preceding polymerization stage. The polymer particles may be introduced into the gas phase reactor as disclosed in EP 1415999 and WO 00/26258.
The polymeric product may be withdrawn from the gas phase reactor either continuously or intermittently. Combinations of these methods may also be used. Continuous withdrawal is disclosed, among others, in WO 00/29452. Intermittent withdrawal is disclosed, among others, in U.S. Pat. No. 4,621,952, EP 188125, EP 250169 and EP 579426.
The top part of the gas phase reactor may include a so called disengagement zone. In such a zone the diameter of the reactor is increased to reduce the gas velocity and allow the particles that are carried from the bed with the fluidization gas to settle back to the bed.
The bed level may be observed by different techniques known in the art. For instance, the pressure difference between the bottom of the reactor and a specific height of the bed may be recorded over the whole length of the reactor and the bed level may be calculated based on the pressure difference values. Such a calculation yields a time-averaged level. It is also possible to use ultrasonic sensors or radioactive sensors. With these methods instantaneous levels may be obtained, which of course may then be averaged over time to obtain time-averaged bed level.
Also antistatic agent(s) may be introduced into the gas phase reactor if needed. Suitable antistatic agents and methods to use them are disclosed, among others, in U.S. Pat. No. 5,026,795, U.S. Pat. No. 4,803,251, U.S. Pat. No. 4,532,311, U.S. Pat. No. 4,855,370 and EP 560035. They are usually polar compounds and include, among others, water, ketones and alcohols.
The reactor may also include a mechanical agitator to further facilitate mixing within the fluidized bed. An example of suitable agitator design is given in EP 707513.
Another type of such reactors is a moving bed reactor.
In the moving bed the catalytic polymerization conditions are different compared to those in the fluidized bed. First, the bed density is higher in the moving bed unit. Furthermore, in order to apply different polymerization conditions use is made of a separation fluidum in order to create different catalytic polymerization conditions. For example, a polymerization may be carried out at a lower concentration of a chaingrowth terminating agent such as hydrogen. Applying a separating fluidum to the moving bed unit results in a separation in reaction conditions between the fluidized bed unit and the moving bed unit. Preferably the separation fluidum is added inside the moving bed, preferably to a level of from 0.1 to 0.7 of the total bed level of the moving bed above the base of the moving bed, and forms a cushion on the moving bed through which cushion of separating fluidum the particulate polymeric material settles on the forming moving bed. The separation fluidum may be a gas or a liquid or a mixture of a gas and a liquid. The separation fluidum may be inert to the catalytic polymerization such as nitrogen and C1-C12-alkane.
The separation fluidum may be reactive such as monomer, comonomer such as C2-C12-alkylene or mixtures thereof. Mixtures of inert and catalytic polymerization reactive separation fluidum may be used as desired.
Preferably, use is made of a separation fluidum which is a liquid which evaporates under the conditions residing during the catalytic polymerization in the moving bed. Accordingly, during evaporation a gas cushion of separating fluidum is formed and at the same time a cooling of the exothermic polymerization reaction occurs with at the same time a much higher reactant concentration when using reactive separation fluidum.
The addition of separation fluidum but also the reactant to both fluidized bed unit and moving bed unit may be such that in the fluidized bed unit and/or in the moving bed unit a condensed mode polymerization occurs which is beneficial to productivity.
It is further preferred when the separation fluidum comprises a polymerization monomer or comonomer or mixture thereof.
WO2004/111095 discloses a reactor system and a process for the catalytic polymerization of olefins. The reactor system comprises a fluidized bed unit and a moving bed unit which are integrated such that the residence time in the fluidized bed unit and the residence time in the moving bed unit could be independently controlled. The outlet of the moving bed unit connected to the fluidized bed unit could be provided with means for controlling the outflow rate of polymer particles from the moving bed unit into the fluidized bed unit.
The means for controlling the outflow of polymer particles are not further described in WO2004/111095.